1. Field of the Invention
The invention relates to an arrangement in which a useful signal is transmitted between at least two device units which each have a separate power supply circuit, the power supply circuit of at least one of said device units being powered from a, preferably electrochemical, energy storage means and the power supply circuit of at least a second one of said device units being powered from a power supply mains.
2. Description of Related Art
Such arrangements are used, for example, in electrical or electronic devices which are remote-controlled through high-frequency transmission of a useful signal and in cordless telephones. When in such an arrangement the power supply from the power supply mains is interrupted, for example as a result of a mains power failure, the device unit which is energized from this power supply mains becomes inoperative. In the example of the cordless telephone this means that operation is not possible although for the rest the device units are in working order. In this case the cordless telephone is, for example, no longer available for emergency calls. Conversely, incoming calls can neither be received. This can be particularly annoying if the failure of the power supply mains is not noticed or is not noticed in due time. Particularly in the United Kingdom it is mandatory to equip the base station of a cordless telephone with an additional battery unit which is connected to the power supply arrangement (mains supply) of the base station of the cordless telephone and which takes over the power supply in case of a mains failure. However, such a battery unit has the disadvantage that a comparatively heavy and bulky device part has to be provided for a generally short fraction of the utilization time of such a cordless telephone. The expense required for this is in conflict with the uncertainty whether at the time of a mains failure the batteries still have enough charge to allow them to actually provide the required backup power supply. This cannot be the case, for example, if it has been forgotten to insert new batteries or if in the course of time the batteries have been discharged by internal leakage currents.